Using a Single Library for Entire Site and When to Use Subsites

Question

I'm hoping a discussion I am having with my coworkers can be solved.  We are trying to decide two things:
Should the entire office be under a single site or should each division have their own site?  Our Office has 5 divisions that contain 17 Branches, one site for business functions and one top-level site for the entire office.
Is it standard practice to create one huge library managed by metadata or should there be multiple libraries?  I tend to think one library will be too hard to manage with regard to metadata and site columns and that libraries should be based on business
functions or subject domains (i.e., SOPs, Quality Documents, Business Documents, Technical Documents, etc).
Currently, several workflows, libraries and lists have already been created under our business functions site.  In order to house everything under a single site, we would need to change the name of the site and move everything under it.  I'm not
sure if changing the name of the existing site will cause problems.
I group these two items together because they really seem related.  With everything under a single site, individual libraries, lists and workflows could get out of hand as everyone tries to manage their own division.  But then so would a single
library for everyone.  The problem with more than one site is that data cannot flow freely between sites (via workflows) and search would be less effective so some are against separate sites.  So knowing one question may affect the answer for the
other.
Any thoughts on our library and site structure would be appreciated.  Thanks.

Answer 1

As always the usual SharePoint consultant cop-out answer of 'it depends' is correct.
Having said that there are certain trends that are quite relevant:
It isn't common to use a single site collection for all sites. It can be done but it often leads to a state where you have a single monstrous site collection which is harder to backup, recover and maintain (see the boundaries and limits documents). The
default behaviour in most cases is to have a central 'shared' site collection then one per <group> where Group is a team that works on the same content, has broadly the same area of interest and access. That might be one for HR, one for the Support team,
one for the Sales team etc.
In terms of libraries there's a wide variation. If there' no difference in views, security and metadata then keeping the library count down is generally a good thing. However using multiple libraries can make life easier and better for users by allowing
more useful views, more secure by locking down libraries rather than using item level permissions (always a nightmare) and allowing more useful alerts and so on.
SharePoint architecture is a science but it's a really, really hard one to formulate into If/Then rules. So, broadly answering your questions I would lean towards multiple site collections and I generally agree with you on point 2, although if there's only
a small number of documents and simple structure then a single library might still make sense.